This invention relates generally to seals for floating roofs in storage tanks for storing petroleum products or other liquid materials.
In such tanks, several problems are encountered with respect to providing a seal between a floating roof in the tank and the tank wall. The floating roof typically floats on the product, and as the product is introduced into the tank to fill the tank and removed therefrom, the roof rises and falls in the tank. Thus, the roof must be spaced at its outer periphery from the tank wall in order to prevent binding of the roof in the tank as the roof rises and falls. This space exposes the product stored in the tank, thus permitting the product to evaporate and be lost to atmosphere and also permitting rain, snow, debris and the like to enter the product through the space. Accordingly, many different types of seals have been devised in the prior art in order to seal this space as the roof rises and falls in order to prevent loss of the product to atmosphere and also to prevent entry of contaminants into the product through the space. Moreover, such seals must not only seal the space to prevent evaporation of the product or contamination thereof, but must also be capable of moving along the inner surface of the tank wall as the roof rises and falls without binding or impeding normal operation of the floating roof.
Examples of some prior art seals are: mechanical seals having steel shoes which are biased against the tank wall and supported from the floating roof, with a fabric seal extending between the steel shoes and the roof; foam filled bags or envelopes which extend in the space defined by the roof and tank wall; and liquid filled tubes suspended or supported in the space between the roof and tank and engaged therebetween. All of these seals are effective to at least a certain degree in preventing evaporative loss of the stored product and also in preventing contamination of the product by rain, snow or debris. Further, these seals are all flexible or yieldable and thus accommodate themselves, at least to a certain degree, to irregularities in the tank shape or surface, thereby maintaining a seal even though the tank may not be perfectly cylindrical or even though weld deformaties or rivet heads and the like are exposed at the tank inner surface. However, all such prior art seals are relatively fixed insofar as their configuration and position is concerned relative to the roof and tank and the space therebetween. In other words, during normal usage the rim space between the roof and tank varies as the roof rises and falls, or even while the roof remains stationary at a predetermined level in the tank, and the seals are either compressed or they expand in the rim space as the rim space changes. However, some such prior art seals are subject to excessive wear at decreased or minimum rim spaces, and at increased or maximum rim spaces, excessive gaps between the seal and the tank wall sometimes occur, thus enabling product to evaporate and be lost to atmosphere or enabling contaminants to enter the product through the gap.
Because of these problems, additional seal means or other arrangements are now being considered to compensate for the deficiencies in the primary seals presently used in the rim space between a floating roof and the tank wall. Recent studies have indicated that faulty seals result in a substantial quantity of the stored product being lost to atmosphere through evaporation, thereby not only depleting the supply of fuel, but also polluting the atmosphere. In view of the serious shortage of energy resources, and particularly petroleum products such as would typically be stored in a tank with which the present invention is concerned, the importance of an effective seal in the rim space between a floating roof and a tank wall is readily apparent. In fact, the California Air Resources Board has recently promulgated a Rule 463 requiring that storage tanks comply with certain parameters regarding loss of product to the atmosphere.
Many efforts are presently being made to comply with these parameters, ranging from expensive and complicated equipment which recovers vapors emitted from the storage tank and pumps them to a station where the vapors are reliquefied and converted to usable energy materials, to the provision of secondary seals used in conjunction with the conventional primary seals.
Moreover, the floating roofs or decks used in such tanks may be used either as the roof itself in an open top tank or a fixed outer roof may be used on the tank, in which case, the floating roof would comprise an inner deck. When the floating roof is used in an open top tank, the seal must be capable of withstanding weather and the like. Further, the aforementioned recent studies have indicated that wind forces play an important part in evaporative loss of the stored product, in that the aerodynamics associated with the tank and floating roof construction are such that wind forces tend to enter the space beneath or adjacent the seal on the downwind side of the tank and move around this annular space defined by the tank, seal, roof and stored product and exit from the space at the upwind side of tank carrying vapors of the stored product into the atmosphere.